The present invention is related to a wave plate, system and methods of making and using same, and more particularly, to a variable quasioptical wave plate that is particularly useful for millimeter wave frequencies, as well as a system including same, and methods of making and using such a wave plate and system.
Wave plates are devices that utilize an anisotropy to change the polarization (direction of the electric field vector) of an incident electromagnetic wave, i.e., to change the direction of the electric field vector. Wave plates frequently are used in optical systems, to induce a change in the polarization of an incident electromagnetic wave, such as light. Such optical wave plates typically are constructed from a birefringent material, i.e., one having different indices of refraction for different incident polarizations. For example, an optical wave plate may be a dielectric plate having different refractive indices in orthogonal x and y directions (with the z direction normal to the surface of the plate in an orthogonal coordinate system). More specifically, a wave plate may be made from a birefringent material such as calcite, for example, whose optic axis is parallel to the surface. In terms of the polarization of the incident wave directed to a wave plate, there are two cases of interest: when the electric field vector of the incident electromagnetic wave is parallel to the optic axis, and when it is perpendicular to the optic axis. The component of the incident wave whose electric vector is parallel to the optic axis is known as the extraordinary wave (or e-wave for short), and the wave component whose electric vector is perpendicular to the optic axis is known as the ordinary wave (o-wave). Each wave or component sees a different index of refraction, ne for the extraordinary wave, and no for the ordinary wave. As a result, upon propagation through a plate of thickness L, the difference in phase between the two wave components is given by
xcex94xcfx86={fraction (2xcfx80/xcex)}(nexe2x88x92no)L,
where xcex is the free-space wavelength. An incident wave that is linearly polarized and whose electric field vector makes an angle of 45xc2x0 relative to the optic axis may be considered to have e-wave and o-wave components having equal magnitudes and phases.
If L is chosen so that xcex94xcfx86=2xcfx80, the device, known as a full-wave plate, has no effect on the polarization of the transmitted electromagnetic wave. If L is chosen so that xcex94xcfx86=xcfx80, the polarization of the transmitted electromagnetic wave is rotated about an axis parallel to the direction of propagation by 90xc2x0 relative to that of the incident wave, and the resulting device is known as a half-wave plate. If L is chosen so that xcex94xcfx86=xcfx80/2, the wave plate induces a 90xc2x0 phase shift between the initially in-phase electric-field components of the incident wave, resulting in a circularly polarized transmitted wave. Such a wave plate is known as a quarter-wave plate. Finally, if L is chosen so that xcex94xcfx86=xcfx80/4, the plate induces a 45xc2x0 shift in phase between the initially in-phase components of the incident wave. Such a wave plate is known as an eighth-wave plate.
Wave plates may be used in quasioptical millimeter-wave systems. However, the wave plates described above are seldom used at millimeter-wave frequencies (frequencies at which the wavelength is between 1 and 10 millimeters) for two primary reasons. First, accurate measurements of the anisotropic properties of dielectrics at millimeter-wave frequencies are very limited. Second, most materials have relatively high losses at millimeter-wave frequencies, limiting their usefulness. This problem is compounded for anisotropic materials, since the loss also tends to be anisotropic, producing an absorption that is dependent on the electric-field polarization.
Wave plates also have been made from isotropic dielectrics by inducing an artificial anisotropy. For example, half- and quarter-wave plates have been made from Rexolite(copyright) (a low loss polymer available from C-LEC Plastics, Inc. of Beverly, N.J., U.S.A.) by machining a series of parallel grooves in the surface of a plate. An incident wave whose electric field is polarized parallel to the grooves will see a different index of refraction than an incident wave whose electric field is polarized perpendicular to the grooves. This technique is effective at low power levels; however, at high power levels the low thermal conductivity of Rexolite(copyright) causes excess heat to accumulate until the plate fails.
For high power levels, wave plates have been constructed of metal plates by fabricating periodic arrays of rectangular or elliptical slots in the plates. For example, see Paul F. Goldsmith, Quasioptical Systems: Gaussian Beam Quasioptical Propagation and Applications (1998). Each slot acts like a waveguide. For a rectangular waveguide of width W and height H (where W greater than H), for example, the phase shift per unit length for the TE10 mode (electric field polarized parallel to H) is different from that for the TE01 mode (electric field polarized parallel to W). A similar relationship exists for an elliptical waveguide. By properly choosing the slot dimensions, the thickness of the plate, and the periodicity of the slots, the desired relative phase shift can be imposed between the orthogonally-polarized components of a normally-incident wave, and the reflected power for each component of the incident wave can be minimized. Such a wave plate, being of all-metal construction, can handle very high power levels, particularly if it is actively cooled around the edges, or includes internal cooling channels.
A slotted metal or metallic wave plate like that just described is difficult and expensive to fabricate, however, as the slots for millimeter-wave wavelengths are too small to be made by conventional machining techniques. Typically, the rectangular or elliptical slots have to be formed using some form of electron-discharge machining (EDM). If wire EDM is used, a hole first is machined where each slot is to be placed, then the EDM wire is threaded through the hole. After cutting the slot to the desired dimensions, the wire is cut and is manually threaded through the next hole. As this technique is very labor intensive, it is not cost effective if more than one or two wave plates are to be constructed.
Another form of EDM uses a mandrel. To construct a wave plate of the type described above, the mandrel has a xe2x80x9cwafflexe2x80x9d pattern of raised rectangular protrusions extending from its surface. The mandrel is then used to xe2x80x9cburnxe2x80x9d the desired pattern into the metal plate. This type of EDM results in gradual deformation of the mandrel, which has to be trimmed after burning part way through the plate and eventually has to be replaced. A wave plate constructed in this manner would likely be less expensive than one constructed using wire EDM, but generally is still cost-prohibitive.
The present invention provides a wave plate that is based on a perforated metallic plate. However, unlike the slotted metallic plates previously used in highpower applications, the wave plate provided by the present invention has circular through-holes and induces an anisotropy by forming holes of a predetermined size in a predetermined pattern. More specifically, by proper choice of the hole diameter and plate thickness, and by different spacing of the holes in respective orthogonal x and y directions, the desired relative phase difference between orthogonally-polarized components is achieved while minimizing the reflected power for both polarization components and maximizing power transmission through the plate.
The present invention replaces the rectangular or elliptical slots with circular holes, eliminating the need for EDM and resulting in significantly lower manufacturing costs. A wave plate made with circular holes can be made with conventional machining techniques, eliminating the need for EDM and resulting in a significant cost advantage over wave plates made with rectangular or elliptical slots. Using conventional machining techniques, the holes can be reamed or drilled, for example, using a numerically-controlled milling machine. These conventional machining techniques also are much faster than EDM. Once the initial one-time set-up costs have been incurred (e.g., tooling, programming the milling machine, etc.), recurring costs are relatively low, significantly lower than for electron-discharge machining.
In an exemplary embodiment, the present invention provides a wave plate for inducing a change in the polarization components of an incident electromagnetic wave that includes a plate having a metal surface and an array of circular through-holes having a predetermined diameter. The diameter of each hole, the thickness of the plate, and the relative positions of the holes combine to change the polarization of the electromagnetic wave as it passes from an incident side of the plate to an outlet side of the plate while minimizing the reflected power.
According to the present invention, such a wave plate may also include one or more of the following features: a wave plate wherein in an orthogonal grid on the surface of the plate, the through-holes are spaced a first distance in an x-direction and are spaced a second distance different from the first distance in a y-direction;
a wave plate wherein the plate is metal;
a wave plate wherein the plate is a nonmetallic material having a metal coating;
a wave plate wherein the plate is substantially flat;
a wave plate wherein the plate has a uniform thickness;
a wave plate wherein the plate has a uniform thickness of about 251 mils (about 6.4 mm);
a wave plate wherein the diameter of the holes is uniform throughout the array of holes;
a wave plate wherein the hole radius is about 39 mils (about 1.0 mm);
a wave plate wherein the hole diameter, plate thickness and hole spacing are selected for frequencies greater than about 20 GHz;
a wave plate wherein the hole diameter, plate thickness and hole spacing are selected for a frequency of about 95 GHz;
a wave plate wherein the nearest-neighbor distance between adjacent holes is uniform in a first direction and the nearest-neighbor distance in a second orthogonal direction is uniform, but the nearest-neighbor distances in the first direction and in the second direction are not the same; and
a wave plate wherein the nearest-neighbor distance between adjacent holes in the first direction is about 103.5 mils (about 2.6 mm), and the nearest-neighbor distance between adjacent holes in the second direction is about 118.0 mils (about 3.0 mm).
The present invention also provides a variable wave plate system that includes a plurality of axially aligned wave plates.
In addition, the present invention provides a method of making a wave plate that includes selecting the hole diameter, the plate thickness and the hole spacing for maximum transmission, desired phase shift and minimum reflection, and forming the holes in the plate. The step of forming the holes may include at least one of machine reaming, electron-discharge machining, and drilling.
The present invention also provides a method of effecting a relative shift in phase between the polarization components of an electromagnetic wave that includes the steps of providing at least one wave plate and directing an electromagnetic wave through the holes in the at least one wave plate.
The present invention further provides a method that includes providing at least two wave plates arranged in parallel, axially aligned, and spaced apart at least two wavelengths, and rotating at least one wave plate about a central axis to vary the change in polarization.
Also provided by the present invention is a variable polarization rotation system. The system includes at least two wave plates with adjacent wave plates spaced from one another by at least two wavelengths of the intended incident electromagnetic wave. For example, the system may include four wave plates.